The present invention relates to systems for analyzing a surface of a rotating recording disc. More specifically, the present invention relates to an apparatus for detecting small defects on the surface of a rotating recording disc using a near field optical head.
In data processing systems, disc drives are often used as direct access storage devices. In such devices, read/write heads are used to write data on or read data from an adjacently rotating hard or flexible disk. To prevent damage to either the disc or the read/write head, it has been recognized that the surface of the disc should be very flat and free of any bumps or the like which might be contacted by the read/write head. Also, the read/write heads have been designed so that they will fly over the surface of the rotating disc with a small fly height which is maintained by a film of air. During its flight, the head undergoes continuous vibration, pitch and roll as the topography of the disc changes beneath the head. If the quality of the disc or the read/write head is poor, occasional rubbing or sharp contact may occur between the disc and the read/write head. Such contact may damage the head or the disc, cause loss of valuable data, or both of these.
Various attempts have been made to provide increased assurance that such undesirable contact between a read/write head and a recording disc does not occur. Rigid manufacturing and quality assurance specifications for both the recording disc and the read/write head have been instituted. Additionally, the art has compensated for the presence of these defects by increasing the fly height of the head, and incorporating correction algorithms into the disc drive circuitry.
Disc inspection for various types of defects, including magnetic, optical and topographic (i.e., delamination, voids inclusions, asperities, etc.) is of critical importance for the increasingly stringent production requirements facing a manufacturer today as smaller drives store more data. Various methods of defect inspection are currently in use. These include optical techniques (fiber interferometry, bulk optic shear interferometry, microISA), magnetic readout (simple screening, HRF, etc.) and mechanical testing. Each of these techniques may play a role in achieving the goal of virtually defect free production of magnetic discs. However, with the market tightening and technical requirements (such as fly height and speed) becoming more exacting and demanding, inspection schemes which are more efficient and accurate are highly advantageous.
There are various types of heads which are currently being used to screen the discs for surface variations or asperities (xe2x80x9csurface variationsxe2x80x9d and xe2x80x9casperitiesxe2x80x9d are intended to have identical meanings as used herein).
One such type is the certifier head. This is usually an MR read head which is used to map the disc for flaws in the magnetic layer. When the certifier head encounters a defect, the impact causes the MR element to heat which results in a change in resistance. This causes the MR element output to signal a super pulse followed by a missing pulse. The certifier head is typically used to scan only a portion of the disc (less than 50%) due to its small size, and generally does so at the same height as the product head. Additionally, defects below the normal product head fly height can easily be encountered by the product head in a stressed condition (for example at increased altitude).
An optical certifying head flies above a disc surface within an evanescent delay length of the disc surface. A light beam is focussed through an objective lens, and further focussed through a (Solid Immersion Lens) SIL. The SIL-focussed light is coupled to the disc surface through near-field coupling. The light is reflected from the disc surface depending on a surface condition of the disc such that the surface condition can be determined. A method of making the optical certifying head and a method of using it are also provided.